Conventionally, a device in which various types of parts are joined or assembled by screw attachment has problems. In cases where the screws are unnecessarily removed by a third party, the device is disassembled or its parts are removed, resulting in that adjustment sections or dangerous sections are exposed, and it becomes difficult to restore the device to its original functioning state, and in addition emergency situations would occur. From such a standpoint, various types of tamperproof screws that cannot easily be tightened or removed by a common commercially marketed screwdriver (screw turning tool) have been conventionally proposed in the past.
One that has been proposed as a tamperproof screw of this type is, for example, a screw in which a plurality of fan-shaped projections that are equally divided by straight lines in the radial direction are formed in the upper surface of the circular head, and this surface is used as a one-way rotational screw surface, so that an ordinary screwdriver cannot be engaged with the screw, and the screw is tightened utilizing the vertical surface formed on the end surface of one of the fan-shaped projections only in cases where a special tool is used. (Japanese Patent Application Laid Open (Kokai) No. 2000-220619).
Furthermore, in the special tool that is used for the above-described tamperproof screw, a circular hollow space with which the circular head of the tamperproof screw engages is formed in the tip end head of the tool main body, and a plurality of fan-shaped projections having a shape that corresponds in a male-female relationship to the fan-shaped projections formed on the circular head of the tamperproof screw are provided inside this circular hollow space.
Furthermore, a special bolt and special nut, which cannot easily be rotated by means of a commonly used tool so that there is no danger of these elements being easily rotated by a third party, and a tightening device, which is comprised of these special bolt and nut, have been proposed (Japanese Patent Application Laid Open (Kokai) No. H11-270529).
In the structure of this special bolt according to the proposal of this prior art, a truncated cone shape head whose external diameter becomes gradually smaller toward the tip end is concentrically formed at one end portion of the shank that has screw grooves formed on other end portion thereof, an engaging hollow space which has the cross-sectional shape of a regular pentagon in which the respective corner portions are formed in a circular arc shape is formed in the tip end of this head so that the hollow space opens toward the tip end surface of the head, and a truncated cone shape projection whose external diameter becomes gradually smaller toward the tip end is formed inside this engaging hollow space.
Accordingly, since the head of the special bolt constructed in this manner has the shape of a truncated cone with a diameter that becomes gradually smaller toward the tip end, even if it is attempted to engage an ordinary tool such as wrench or spanner, etc. with this head, this tool readily slips off toward the tip end without engaging with the head. In other words, a tamperproof effect against third parties can be effectively manifested.
Furthermore, the special tool that can be engaged with this special bolt so that the bolt can be rotated has a structure in which the tool is comprised of a tool main body, which has the shape of a hexagonal column, and a head, which is disposed on one end of this tool main body. The head is in the shape of a regular pentagonal column which has outer circumferential faces that match the inner circumferential surfaces of the engaging hollow space in the head of the special bolt. A through-hole having an internal diameter that allows engagement with the projection on the head of the special bolt is formed in the axial central portion of this head. The through-hole has a constant internal diameter over the entire length of the special tool, and the axial central portion of the tool main body also passes through. Accordingly, in this special tool, the head can be engaged with the engaging hollow space of the special bolt, and the special bolt can easily be rotated as a result.
Furthermore, in common combinations of a screw and a screwdriver bit used in the past, a cruciform groove is formed in the head of the screw, and this cruciform groove is formed so that substantially constant inclined portions extend respectively from the respective open end edge portion toward the central portion of the neck portion of the screw, and a substantially conical bottom surface with a gradual inclination is formed in the bottom of this cruciform groove. Meanwhile, a conventional screwdriver bit comprises respective tip end vane portions that engage with the cruciform groove of the screw and has a structure in which projections are formed respectively to extend so as to match the shapes of the inclined portions that extend from the open end edge portion of the cruciform groove toward the central portion of the neck portion of the screw.
In the combination of the conventional screw and screwdriver bit having such structures described above, tapered side wall portions formed on the respective tip end vane portions of the screwdriver bit contact and engage with tapered side wall portions formed in the cruciform groove of the screw; accordingly, when the screwdriver bit is rotated in a particular direction, the state of contact between the screwdriver bit and cruciform groove is in tapered contact as a whole. Consequently, a so called “come-out” phenomenon occurs, in which the tip end of the screwdriver bit tends to jump to the outside along the inclined surfaces of the inclined portions of the cruciform groove. Accordingly, when this come-out phenomenon occurs, not only are the open end edge portion of the cruciform groove of the screw damaged, but also the widening of such damage causes the come-out phenomenon to become even more prevalent, and problems such as the impossibility of performing screw tightening work, etc. arise.
From such a standpoint, the applicant of the present invention previously succeeded in developing a new screw in which the come-out phenomenon of the screwdriver bit is assuredly prevented, the strength of the screw is high so that damage to the screw is conspicuously reduced, and a balanced torque with respect to the screw is obtained, by forming step portions that are oriented substantially at right angles in intermediate point of the respective inclined portions of the bit engaging grooves, and causing a portion of the screwdriver bit to engage with these step portions, in a screw in which bit engaging grooves consisting of a cruciform groove is formed in the screw head, specified inclined portions are formed so as to be oriented toward the central portion of the neck portion of the screw from the end edge portions of this bit engaging grooves, and the central portion where these inclined portions meet has a substantially conical bottom surface (Japanese Patent Application Laid Open (Kokai) No. H9-177743, Japanese Patent Application Laid Open (Kokai) No. H11-236910, Japanese Patent Application Laid Open (Kokai) No. H11-311226, Japanese Patent Application Laid Open (Kokai) No. 2000-108043, etc.).
In this case, furthermore, for the screwdriver bit, it was possible to form a structure most suited to the bit engaging grooves of the screw by way of forming vane portions having substantially perpendicular end edge portions that engage with the step portions formed in intermediate point of the bit engaging grooves of the screw and by way of forming, on the respective vane portions, protruding portions whose tip ends respectively extend so as to conform to the shape of the groove portions that extend from the step portions.
Accordingly, it was ascertained that if a combination of a screw and screwdriver bit having the above-described structures is used, the formation of the bit engaging grooves can easily be accomplished without lowering the strength of the screw, especially in cases where this combination is applied to screws used in compact light-weight precision instruments. Furthermore, it was found that the come-out phenomenon of the screwdriver bit is assuredly prevented and that torque transmission that is balanced with respect to the screw is accomplished, resulting in that proper and safe screw tightening work can always be accomplished.
However, compared to the conventional ordinary combination of screw and screwdriver, the above-described tamperproof screw has extremely complicated special shapes in the respective structures of both the engaging portion of the screw head and the tip end portion of the screwdriver bit that engages with this engaging portion. Accordingly, the manufacture of such a screw and a screwdriver bit requires considerable effort, and the manufacturing cost also becomes high. Furthermore, in this combination of tamperproof screw and screwdriver bit, absolutely no consideration is given to the come-out phenomenon. Accordingly, even if the combination is effective in preventing tampering with the tamperproof screw by means of an ordinary screwdriver, there is no guarantee that tightening and removal of the screw by the proposed combination of tamperproof screw and screwdriver is smoothly accomplished without any come-out phenomenon; and on the contrary, the resulting shape tends to allow the come-out phenomenon to occur.
In view of the above, as a result of diligent research and investigations, the inventor of the present application succeeded in obtaining a new tamperproof screw in which a screw is constructed by forming bit engaging grooves consisting of a cruciform groove in the screw head, by forming inclined portions that have a specified angle of inclination toward the central portion of the neck portion of the screw from the open end edge portions of this bit engaging grooves, by forming respective step portions that are substantially perpendicular in cross section in intermediate point of the inclined portions, and by forming a substantially conical bottom surface in the central portion where the inclined portions meet; and the tamperproof screw is constructed by forming an inverted truncated cone shape hole portion which reaches the bottom surface as a continuation, except for the step portions, of the inclined portions so that the open end edge portions of the bit engaging grooves takes the maximum diameter of the hole portion; so that tightening and removal of the screw by the conventional ordinary plus type screwdriver is assuredly prevented due to the presence of the inverted truncated cone shape hole portion.
On the other hand, for the tamperproof screw that has the structure described above, a screwdriver bit is provided in which vane portions having substantially perpendicular end edge portions that engage with the step portions formed in intermediate point of the inclined portions of bit engaging grooves of the above-described tamperproof screw are respectively formed, and the tip ends of the respective vane portions are formed as protruding portions that are respectively extended so as to conform to the shape of the inverted truncated cone shape hole portion and the inclined portions extending toward the central portion of the neck portion of the screw from the step portions. As a result, it was ensured that tightening and removal of the above-described tamperproof screw could be easily and assuredly accomplished in a quick and simple manner without causing any “come-out” from the bit engaging grooves of the screw.
Furthermore, in the manufacture of the tamperproof screw having the structure described above, a header punch is provided that is comprised of projecting portions, which have substantially perpendicular end edge portions and form the perpendicular and recessed wall portions and step portions in the end edge portions of bit engaging grooves in the screw head of the tamperproof screw, and a conical projecting portion, which forms an inverted truncated cone shape hole portion and inclined portions in the central portion of the bit engaging grooves and to form a substantially conical bottom surface in the tamperproof screw. As a result, it was ensured that the manufacture of the above-described tamperproof screw could be accomplished simply and easily and at a low cost.
Furthermore, as an alternative of the above-described tamperproof screw, a structure can be employed in which an inverted truncated cone shape hole portion that reaches the bottom surface is formed as a continuation, except for the step portions, of the inclined portions so that the open end edge portions of the bit engaging grooves take a maximum diameter of the inverted truncated cone shape hole portion, and a projecting portion is formed so as to protrude from the central portion of the bottom surface to a position above the step portions of the bit engaging grooves. As a result, it was confirmed that tightening and removal of the screw by means of a conventional ordinary plus type screwdriver or minus type screwdriver could also be assuredly prevented because of the presence of the projecting portion and inverted truncated cone shape hole portion.
In this case, for a screwdriver bit that is used for the above tamperproof screw, it has such a structure that a protruding portion is formed by extending the tip ends of the respective vane portions so as to conform to the shape of the inverted truncated cone shape hole portion and the inclined portions that extend from the step portions toward the central portion of the neck portion of the screw, and a hollow space is formed in the center of the tip end of the protruding portion so that the projecting portion is guided thereinto. With this structure, it was confirmed that tightening and removal of the tamperproof screw could be performed easily and assuredly in a quick and simple manner without causing any “come-out” from the bit engaging grooves of the screw.
Furthermore, it was confirmed that the manufacture of the above-described tamperproof screw could be accomplished simply and easily and at a low cost by a header punch that has a structure including a conical projecting portion, which forms an inverted truncated cone shape hole portion and inclined portions in the central portion of the bit engaging grooves and to form a substantially conical bottom surface in the inverted truncated cone shape hole portion, and a cavity portion, which is provided in the center of the tip end of the conical projecting portion and forms a projecting portion in the inverted truncated cone shape hole portion of the tamperproof screw.
Accordingly, the object of the present invention is to provide: a tamperproof screw in which tightening thereof can be assuredly prevented by improving the shape of the bit engaging grooves based upon the structure of a conventional ordinary screw so that coupling by means of a conventional ordinary screwdriver is prevented by a simple structure, a screwdriver bit in which coupling with the tamperproof screw can be accomplished quickly and easily without causing the phenomenon of “come-out” during the tightening of the screw by improving the shape of the tip end of the bit based upon the structure of a conventional screwdriver bit, and a header punch used for manufacturing the tamperproof screw simply and easily and at a low cost.